Skip to content Your generator is a pipe, not a tank
Almost every sizing mistake we see starts with the same mental picture: people imagine a generator as a fuel tank of electricity that fills up and drains down.
It does not work that way. A generator produces power in real time and delivers exactly what the house is asking for at that instant, up to a hard ceiling. Think of a pipe with a fixed width. If the house asks for more than the pipe can pass, nothing gets saved for later and nothing gets borrowed from earlier. The generator simply cannot deliver it, and something has to give.
That single shift in thinking, from tank to pipe, explains most of what people notice when they run on generator power. It explains the flicker when the well pump kicks on. It explains why a sauna heater or an EV charger is harder on a generator than a much larger looking appliance that only runs for ninety seconds.
Capacity is a rate of delivery, not a reserve. Unused output from the last ten minutes does not carry forward into the next ten seconds.
Every appliance draws power in one of three shapes
Once you picture the pipe, the next question is what each appliance is doing inside it. Nameplate wattage alone will mislead you, because two appliances with the same rating can behave completely differently. What matters is the shape of the draw over time.
Same chart, three very different problems. The red trace crosses the rating and the navy trace never does, yet the navy trace is the one that consumes your headroom all evening.
Motor loads spike, then settle
Anything with a motor that has to break away from a standstill pulls a large inrush current for a fraction of a second. Well pumps, air conditioning compressors, air compressors, table saws, and refrigerators all do this. The surge can be several times the running draw, and it is the number that determines whether the generator can start the load at all.
Resistive heat is flat and relentless
Electric heat has no moving parts to get spinning, so there is no surge. That sounds like the easy case. It is harder in a different way. A sauna heater, a water heater element, a baseboard heater, or an EV charger pulls its full rated power for the entire time it is calling, which is most of the heat up and a good share of the session. A load like this sits in the pipe with its hand open.
Cycling loads come and go
A refrigerator or a chest freezer runs a few minutes out of every twenty. Individually they are small. The reason we still count them is that their compressor surge can land at the same instant something else starts, and coincidence is what actually trips generators.
The plumbing example people miss. Flushing a toilet or taking a shower uses no electricity at all on city water. On a private well, which describes a lot of properties out in the hills, both of those actions are what start the pump. The load you feel is never the fixture. It is the motor the fixture wakes up.
What the common stuff in a house actually pulls
These are typical ranges for residential equipment. Treat them as a map for thinking, not as a substitute for a load calculation. Your actual numbers come off the nameplate on your specific equipment, and a proper sizing uses those.
| Appliance | Typical running watts | Startup surge | Draw shape |
|---|---|---|---|
| LED lighting, whole house | 150 to 400 | None | Steady |
| TV and home electronics | 100 to 300 | None | Steady |
| Refrigerator | 150 to 800 | 1,200 to 2,200 | Cycling |
| Chest freezer | 100 to 500 | 800 to 1,500 | Cycling |
| Furnace blower motor | 600 to 900 | 1,500 to 2,400 | Surge |
| Sump pump | 800 to 1,050 | 1,600 to 3,000 | Surge |
| Well pump, 1/2 HP | 800 to 1,000 | 2,000 to 3,000 | Surge |
| Well pump, 1 HP | 1,800 to 2,200 | 4,000 to 6,000 | Surge |
| Vacuum cleaner | 700 to 1,400 | 1,400 to 2,000 | Surge |
| Dishwasher, heated cycle | 1,200 to 1,500 | Minimal | Steady |
| Microwave | 1,000 to 1,800 | Minimal | Steady |
| Garage air compressor | 1,500 to 2,000 | 3,000 to 4,500 | Surge |
| Table saw or miter saw | 1,500 to 1,800 | 3,000 to 4,500 | Surge |
| Central AC or heat pump, 3 ton | 3,500 to 5,000 | 10,000 to 15,000 without a soft starter | Surge |
| Electric water heater, tank | 4,500 | None | Steady |
| Electric clothes dryer | 5,000 to 5,500 | Minimal | Steady |
| Electric range, multiple elements plus oven | 8,000 to 12,000 | None | Steady |
| Residential electric sauna heater, 6 kW to 8 kW | 6,000 to 8,000 | None | Steady |
| Level 2 EV charger, 30 A to 48 A | 7,200 to 11,500 | None | Steady |
| Tankless electric water heater, whole house | 18,000 to 28,000 | None | Steady |
Read the bottom four rows again. Not one of them has a surge, and every one of them is capable of eating an entire mid size home standby generator by itself. That is the whole point of the pipe. A surge is a moment you have to survive. A continuous load is a width you have to permanently give up.
Why the lights dim for a second
When a motor breaks away, it demands far more current than it will need a moment later. The engine has to physically speed back up and the voltage regulator has to catch it. For a fraction of a second, voltage sags, and the filaments and drivers in your lighting show it as a dip.
A brief dip on startup is normal generator behavior and does not mean anything is wrong. What is worth a phone call is a dip that is getting deeper, lasting longer, or happening when nothing large is starting. That usually points at a load that has grown past what the machine was sized for, a failing capacitor or bearing in a motor, or a connection problem.
The base load barely moves. One continuous load is what decides whether the whole picture fits.
How this turns into a generator size
A real sizing exercise is not a matter of adding every nameplate in the house together. It comes down to four questions:
- What has to run at the same time? Nobody runs the dryer, the range, and the sauna simultaneously in an outage. Honest simultaneity is what sets the number.
- What is the single largest starting surge? Usually the AC compressor or the deepest well pump. The generator has to survive that instant on top of whatever else is already running.
- What runs continuously? These get counted at full value with no diversity credit, every time.
- What can be managed instead of sized for? Load shed modules, soft starters on compressors, and a lockout on the EV charger or sauna circuit during utility outages often get a house onto a smaller and considerably cheaper machine.
That last point is where the money is. We have seen plenty of homes where the difference between an oversized unit and a right sized one was a single load management decision, made before anything was ordered.
